The invention is directed to a method for making an optical fiber having optical properties that systematically vary along its length. This method is particularly useful for making dispersion managed (DM) single-mode optical waveguide fibers.
The potentially high bandwidth of single-mode optical fibers can be realized only if the system design is optimized so that the total dispersion is equal to zero or nearly equal to zero at the operating wavelength. The term xe2x80x9cdispersionxe2x80x9d refers to pulse broadening and is expressed in ps/nm-km. xe2x80x9cDispersion Productxe2x80x9d refers to dispersion times length and is expressed in ps/nm.
When telecommunications networks employ multiple channel communications or wavelength division multiplexing, the system can experience a loss due to four wave mixing. This loss occurs when the signal wavelength is at or near the zero dispersion wavelength of the optical transmission fiber. This has necessitated the exploration of waveguide fiber designs which can minimize signal degradation that results from this non-linear waveguide effect. A dilemma arises in the design of a waveguide fiber to minimize four wave mixing while maintaining characteristics required for systems which have long spacing between regenerators. That is, in order to substantially eliminate four wave mixing, the waveguide fiber should not be operated near its zero of total dispersion, because four wave mixing occurs when waveguide dispersion is low, i.e., less than about 0.5 ps/nm-km. On the other hand, signals having a wavelength away from the zero of total dispersion of the waveguide are degraded because of the presence of the total dispersion.
One strategy that has been proposed to overcome this dilemma is to construct a system using cabled waveguide fiber segments some of which have a positive total dispersion and some of which have a negative total dispersion. If the length weighted average of dispersion for all the cable segments is close to zero, the regenerator spacing can be large. However, the signal essentially never passes through a waveguide length where the dispersion is close to zero, so that four wave mixing is prevented.
The problem with this strategy is that each link between regenerators must be tailored to give the required length weighted average of dispersion. Maintaining cable dispersion identity from cabling plant through to installation is an undesirable added task and a source of error. Further, the need to provide not only the proper dispersion, but also the proper length of cable having that dispersion, increases the difficulty of manufacture and leads to increased system cost. A further problem arises when one considers the need for replacement cables.
Those problems are overcome by the optical fiber disclosed in U.S. patent application Ser. No. 08/584,868 (Berkey et al.) filed Jan. 11, 1996. In accordance with the teachings of the Berkey et al. application, each individual fiber is made to be a self contained dispersion managed system. A pre-selected, length weighted average of total dispersion, i.e., total dispersion product, is designed into each waveguide fiber. Each waveguide fiber is interchangeable with any other waveguide fiber designed for that system link. Thus, the cabled waveguide fibers all have essentially identical dispersion product characteristics, and there is no need to assign a particular set of cables to a particular part of the system. Power penalty due to four wave mixing is essentially eliminated, or reduced to a pre-selected level, while total link dispersion is held to a pre-selected value, which may be a value substantially equal to zero.
In accordance with the Berkey et al. patent application, the dispersion of a DM fiber varies between a range of positive values and a range of negative values along the waveguide length. The dispersion product, expressed as ps/nm, of a particular length, l, is the product (D ps/nm-km*l km). A positive number of ps/nm will cancel an equal negative number of ps/nm. In general, the dispersion associated with a length li may vary from point to point along li. That is, the dispersion Di lies within a pre-determined range of dispersions, but may vary from point to point along li. To express the contribution of li to the dispersion product, expressed in ps/nm, li is made up of segments dli over which the associated total dispersion Di is essentially constant. Then the sum of products dli*Di characterizes the dispersion product contribution of li. Note that, in the limit where dli approaches zero, the sum of products dli*Di is simply the integral of dli*Di over the length li. If the dispersion is essentially constant over sub-length li, then the sum of products is simply li*Di.
The dispersion of the overall waveguide fiber length is managed by controlling the dispersion Di of each segment dli, so that the sum of the products Di*dli is equal to a pre-selected value over a wavelength range wherein signals may be multiplexed. For high rate systems having long regenerator spacing, the wavelength range in the low attenuation window from about 1525 nm to 1565 nm may be advantageously chosen. In this case, the sum of the dispersion products for the DM fiber would have to be targeted at zero over that range of wavelengths. The Di magnitudes are held above 0.5 ps/nm-km to substantially prevent four wave mixing and below about 20 ps/nm-km so that overly large swings in the waveguide fiber parameters are not required.
The length over which a given total dispersion persists is generally greater than about 0.1 km. This lower length limit reduces the power penalty (see FIG. 5), and simplifies the manufacturing process.
The period of a DM single-mode waveguide is defined as a first length having a total dispersion which is within a first range, plus a second length having a dispersion which is in a second range, wherein the first and second ranges are of opposite sign, plus a transition length over which the dispersion makes a transition between the first and second range. To avoid four wave mixing and any associated power penalty over the transition length, it is advantageous to keep the part of the transition length which has an associated total dispersion less than about 0.5 ps/nm-km as short as possible.
If the transition regions between the regions of higher and lower dispersion are too long, the dispersion in the central portions of the transition regions will be near zero for some finite length of fiber. This will result in some power penalty due to four wave mixing. The longer the transition regions are, the higher the power penalty. The transition regions should therefore be sufficiently sharp that the fiber power penalty does not cause the total system power penalty to exceed the allocated power penalty budget.
A primary requirement of a process for making DM fibers is that it be able to form short transition regions. Moreover, the process of making the DM fiber should not be one that itself induces an excess loss that is unrelated to four wave mixing. Also, the process should be simple and be sufficiently flexible that it can be implemented with a variety of fiber designs and materials. Thus, the DM fiber must be a unitary fiber that is formed by drawing a draw preform or draw blank that includes sections that will form the fiber sections of different dispersion. Such a unitary fiber does not include splices between separately drawn fiber sections, as each splice would introduce additional loss. Ideally, the total attenuation of the unitary fiber is no greater than the composite of the weighted attenuation of each of the serially disposed sections of which it is formed.
An attempt was made to form a DM fiber core rod by fusing together core cane sections by the lathe and torch method. In addition to being difficult to implement, that method suffered from core misalignment, and the flame-caused core wetting problems.
Therefore, an object of the invention is to provide an optical fiber having distinctly different optical characteristics along its length and an improved method for making such a fiber. Another object is to provide a method for making optical fiber of the aforementioned type wherein the transition lengths between sections of different characteristics are very short. A further object is to provide a method for making fiber of the aforementioned type wherein the attenuation is sufficiently low for use as long distance transmission fiber. Another object is to provide a method for making low loss single-mode DM optical fiber having short transition lengths. Yet another object is to provide a method for making optical fibers exhibiting low polarization mode dispersion.
One aspect of the invention concerns a method of making an optical fiber preform. Briefly, the method comprises the following steps. A coating of cladding glass particles is deposited on the outer surface of a cladding glass tube, and a plurality of tablets is inserted into the cladding glass tube. At least one optical characteristic of at least one of the tablets in the tube is different than that of an adjacent tablet, and each tablet has at least a central region of core glass. While the coated assembly is heated to a temperature less than the sintering temperature of the cladding glass particles, a centerline gas is flowed through the tube. The centerline gas is selected from the group consisting of pure chlorine and chlorine mixed with a diluent gas. Thereafter, the coated assembly is heated to sinter the coating, thereby generating a radially-inwardly directed force that causes the tube to collapse onto and fuse to the tablets, and causing the cladding glass tube to shrink longitudinally, whereby adjacent tablets are urged toward one another and are fused to one another.
A further aspect of the invention concerns a unitary optical fiber that results from the above-described method. The fiber comprises a plurality of serially disposed optical fiber sections, each fiber section having a glass core and a glass outer cladding. The core of a first fiber section is different from the core of each fiber section that is adjacent to the first section. The cladding of the first fiber section is identical to the cladding of the adjacent fiber sections. Between each two adjacent fiber sections is a transition region, the length of which is less than 10 meters.